The invention relates to a mechanically-commutated DC motor. In particular, it relates to a mechanically-commutated DC motor in which the stator assembly comprises the windings and the rotor assembly comprises the permanent magnet.
A conventional DC brush motor 100, shown in FIG. 7, gets its name from the brush commutation system, which includes stationary brushes and rotating commutator bars. In a permanent magnet DC brush motor, the stator assembly comprises the permanent magnet and the rotor assembly comprises the windings. The stator 110 of a permanent magnet DC motor will have two or more permanent magnet pole pieces 110a and 110b. The opposite polarities of the energized winding 102 and the stator magnets attract, causing the rotor to rotate until it is aligned with the stator 110. Just as the rotor reaches alignment, the brushes 114 move across the commutator bars 112 and energize the next winding. The commutator bars 112 and brushes 114 result in the reversal of current being made automatically, so the rotor continues to turn in the same direction. The integrated commutation system results in permanent magnet DC brush motors being cost effective and easy to control.
A conventional DC brushless motor 100xe2x80x2 is shown in FIG. 8. Compared to a DC brush motor, a conventional DC brushless motor has an xe2x80x9cinside-outxe2x80x9d configuration. In other words, the permanent magnet 110xe2x80x2 becomes the rotating part and the windings 102xe2x80x2 are placed on the stator poles 110xe2x80x2. Unlike a conventional DC brush motor, a DC brushless motor 100xe2x80x2 cannot be driven by simply connecting it to a source of direct current. An electronic amplifier or driver 112xe2x80x2 is used to perform the commutation electronically in response to low-level signals from an optical or hall-effect sensor 114xe2x80x2. The current in the external circuit must be reversed at defined rotor positions. Hence, the DC brushless motor 100xe2x80x2 is actually being driven by an alternating current through an electronic amplifier or driver 116xe2x80x2, which is connected directly to a direct current source.
The xe2x80x9cinside-outxe2x80x9d configuration commonly used in DC brushless motors improves motor thermal characteristics and has better flux distribution, resulting in a better torque-to-mass ratio.
It is therefore a primary object of the present invention to provide a mechanically-commutated DC motor having the cost effectiveness and ease of control of conventional DC brush motors and the improved motor thermal characteristics, flux distribution, and torque-to-mass ratio characteristics of conventional DC brushless motors.
This and other objects of the invention are achieved by the provision of a DC motor having a mechanical commutation system and an xe2x80x9cinside-outxe2x80x9d configuration, in which the stator assembly comprises the windings and the rotor assembly comprises the permanent magnet. The mechanically commutated DC motor comprises a rotor assembly, a stator assembly, a plurality of terminating conductive pads, and first and second stationary power supply terminals (one of which is positive and one of which is negative). The stator assembly includes a plurality of lamination stacks positioned around a central aperture, a plurality of coil bobbins each being positioned on a corresponding lamination stack, and a plurality of copper coils, each coil being wound on a corresponding bobbin. The rotor assembly includes a rotor body with a permanent magnet ring, a non-conductive timing cam integrated into one face of the rotor body, and a shaft. The rotor body is housed in the aperture of the stator assembly. The permanent magnet ring is radially magnetized in an alternating polarity pattern with at least one pair of poles.
The mechanical commutation system comprises the timing cam, the power supply terminals, and the conductive pads. In general, if there are c stator coils, there are a corresponding number c of conductive pads. Each conductive pad has first and second flexible, inwardly-extending contacts selectively movable into and out of electrical contact with the positive and negative power supply terminals, respectively, in response to the action of the timing cam.
In one aspect of the invention, the stator assembly has three coils connected in a delta configuration, and the rotor body has two magnetized pole pairs.
In another aspect of the invention, the two power supply terminals are concentric, centered on the rotational axis of the rotor assembly, and positioned facing the timing cam. The conductive pads are positioned circumferentially around the outer of the two power supply terminals, with the flexible contacts extending into the aperture of the stator body and being interposed between the timing cam and the power supply terminals. Adjacent ends of adjacent coils terminate on a common conductive pad, so that the ends of each coil terminate on two different conductive pads, and each conductive pad receives the ends of two different coils.